For both environmental and cost reasons, it is important to reduce energy use wherever possible. One major source of wasted energy is lighting, with many electrical lights often being left on unnecessarily. Occupancy detectors are widely used to turn lights on and off automatically when a person enters and leaves a room, and so conserve energy by ensuring that an unoccupied room is not unnecessarily illuminated.
Typically, occupancy detectors work by detecting whether a person is in an area using PIR, microwave or ultrasonic sensors. They can be found in two basic forms: a presence detector, which turns lights on automatically when an occupant is detected, and then turns the lights off after an area is vacated; or an absence detector which requires a manual switch to turn lights on, but similarly turns lights off automatically after an area is vacated.
Normally occupancy detectors are either wall mounted or ceiling mounted. In a typical retrofit application, the lights are already turned on by a wall switch. Replacing this switch with a wall mounted sensor requires no extra wiring. However it is seldom the best place to put an occupancy sensor to give it coverage of the whole area, so there may be dead spots in the detection range. The best place to put an occupancy sensor to give maximum detection is usually on the ceiling, but retrofitting a sensor to the ceiling will always require some extra wiring in order to switch the lighting circuit.
Wall mounted occupancy sensors present a further complication in that most installations will only have Live, Switched Live and Earth wires available at the light switch, with no Neutral connection. Without a Neutral connection it is difficult to generate power. Typically, existing wall mounted occupancy sensors will derive their power source through the lighting load or via Earth, but both of these methods impose a limit on the amount of current that can be drawn.
Therefore there exists a clear need for a lighting control system which is easy to retrofit in an existing building, while also giving the benefits of ceiling-mounted detection.